Prior art extrusion methods have used double-webbed mandrel dies for extrusion of hollow bodies, such as polyvinyl chloride (PVC) hard, compact, and foamed pipes. The double-webbed mandrel die has an inner collar, an outer collar, and a series of webs, which radially join the inner collar and the outer collar.
Double-webbed mandrel dies for production of pipe are known, for example, from a brochure of Battenfeld Extrusionstechnik GmbH entitled “Pipe Extrusion Systems” dated October 2000. Double-webbed mandrel dies are also known in U.S. Pat. No. 4,509,907, issued to Ratheiser on Apr. 9, 1985, the entire teachings of which are incorporated herein by reference.
Prior art dies shape the outer and the inner contour of the pipe using a sleeve and a mandrel. The sleeve and the mandrel are joined together to withstand the wall friction forces and the resulting pressure forces of the extrudate (melt). The mandrel is joined to the sleeve by a series of webs. However, the pressure buildup in the melt is so great that adequate pipe quality is only achieved with sufficient subsequent compression and a limiting of the web height.
In known double-webbed pipe mandrel dies, the connection between the sleeve and the mandrel is provided by two sets of webs, which are staggered in a circumferential direction and joined by a middle collar. This technique offers two advantages. First, the pressure buildup in the melt is not as severe because less compression work is needed and a larger annular gap between sleeve and mandrel can be created. Furthermore, this allows for excellent control of wall thickness of the pipe when using a high production rate, there are no continuous web markings, and the mechanical properties of the pipe are increased when using this type of double-webbed mandrel.